Many natural and man-made gases comprise a variety of different hydrocarbons, and numerous methods and configurations are known in the art to produce commercially relevant fractions from such gases. Among other processes, cryogenic gas separation (see e.g., U.S. Pat. No. 4,157,904 to Campbell et al., U.S. Pat. No. 4,690,702 to Paradowski et al., or U.S. Pat. No. 5,275,005 to Campbell et al.) has become a preferred method of gas separation.
In a typical gas separation process, a feed gas stream under pressure is cooled by heat exchanger and as the gas cools, liquids condense from the cooled gas. The liquids are then expanded and fractionated in a distillation column (e.g., de-ethanizer or de-methanizer) to separate residual components such as methane, nitrogen and other volatile gases as overhead vapor from the desired C2, C3 and heavier components. In some configurations, uncondensed cooled feed gas is expanded to condense additional liquid, which may subsequently be employed as C2 and C3 absorbing agent in an absorber. Various improvements on the basic concept of cryogenic gas separation have been developed.
For example, Rambo et al. describe in U.S. Pat. No. 5,890,378 a system in which (a) the absorber is refluxed, (b) in which the de-ethanizer condenser provides the reflux for both the absorber and the de-ethanizer while the cooling requirements are met using a turboexpander, and (c) in which the absorber and the de-ethanizer operate at substantially the same pressure. Although Rambo's configuration advantageously reduces capital cost for equipment associated with providing reflux for the absorption section and the de-ethanizer, propane recovery significantly decreases as the operating pressure in the absorber rises, especially at a pressure above 500 psig, where separation of ethane from propane in the de-ethanizer becomes increasingly difficult. Consequently, Rambo's system is generally limited by the upper operating limit of the de-ethanizer pressure. Increasing of the absorber pressure while maintaining desirable propane recovery becomes difficult, if not impossible in Rambo's process configuration. Moreover, operating the absorber and de-ethanizer at a pressure at or below 500 psig typically necessitates higher residue gas recompression, thereby incurring relatively high operating cost.
To circumvent at least some of the problems associated with relatively high cost associated with residue gas recompression, Sorensen describes in U.S. Pat. No. 5,953,935 a plant configuration in which an additional fractionation column is included. The absorber reflux in Sorensen's plant configuration is produced by compressing, cooling, and Joule Thomson expansion of a slipstream of feed gas. Although Sorensen's configuration generally provides an improved propane recovery with substantially no increase in plant residue compression horsepower, propane recovery significantly decreases as the operating pressure in the absorber rises, especially at a pressure above about 500 psig. Furthermore, ethane recovery using such known systems designed for propane recovery is normally limited to about 20% recovery.
In order to improve ethane recovery with a low CO2 content in the ethane product, Campbell describes in U.S. Pat. No. 6,182,469 a tower reboiling scheme in which one or more tower liquid distillation streams from a point higher in the absorber are employed for stripping of undesirable components (e.g., carbon dioxide in a demethanizer). Campbell's scheme typically requires over-stripping of the ethane product, and CO2 removal is generally limited to about 6%. Moreover, additional CO2 removal using Campbell's process will significantly reduce ethane recovery, and increase power consumption. Furthermore, and especially where the ethane product is used for chemical production, the product in Campbell's configuration typically requires further treatment to remove CO2 to or below a level of 500 ppmv, which often requires substantial capital and operating expenditure.
Although there are various configurations and processes for improved propane and ethane recovery known in the art, all or almost all of them, suffer from one or more disadvantage. Therefore, there is still a need to provide improved methods and compositions for high propane recovery processes and configurations.